Carrier aggregation (CA) using up to 5 carriers of the same frame structure is supported in Long Term Evolution-Advanced Release 10 (LTE-Rel-10) as can be appreciated from, for example, 3GPP TS 36.211, v10.7.0. More LTE capacity is needed due to interference and the volume of data delivered as the number of LTE devices increases. 3GPP is considering supporting wider spectrum bands at the user equipment (UE) side to boost the peak data rate performance by standardizing enhanced CA using up to 32 component carriers (CC) in the C-band (3.4 to 4.2 GHz) licensed band and 5 GHz (with approximately 500 MHz of unlicensed spectrum) to provide more resources for data capabilities and better manage interference.
In LTE Rel-8, 3GPP TS 36.211, v8.9.0, a single layer Physical Uplink Shared channel (PUSCH) is supported. When Uplink Control Information (UCI) is due in the same subframe as a scheduled PUSCH, the Uplink Control Information is multiplexed with data. More specifically, the number of resource elements (RE) for Hybrid Automatic Repeat Request (HARQ) Acknowledgement/Negative Acknowledgement (ACK/NACK) piggybacked on PUSCH has an upper bound of 4 DFT-S-OFDM symbols near to a reference signal (RS) for better channel estimation. For up to a 21-bit ACK/NACK, such an upper boundary would be sufficient to provide reliable communications.
In RAN1 #81, it was agreed that the maximum ACK/NACK codebook size for 32 downlink (DL) component carriers (CC) is at least 128 bits for Time Division Duplex (TDD) and 64 bits for Frequency Division Duplex (FDD). Additionally, the ACK/NACK payload was increased further accounting for at least an 8-bit Cyclic Redundancy Check (CRC) attachment. Considering the substantially larger ACK/NACK payload size, a 3 to 6 fold increase, the available REs in 4 DFT-S-OFDM symbols might become insufficient for ACK/NACK multiplexing on PUSCH to meet a predetermined reliability.